Method and device for controlling an exhaust treatment system

ABSTRACT

A device and a method for controlling an exhaust-gas aftertreatment system, in particular for an internal combustion engine, is described. The exhaust-gas aftertreatment system includes at least one catalytic converter. A preselectable quantity of reducing agent is supplied to the exhaust-gas aftertreatment system as a function of the state of the internal combustion engine and/or the exhaust-gas aftertreatment system. The quantity of reducing agent supplied is adjusted.

BACKGROUND INFORMATION

[0001] The present invention relates to a method and a device forcontrolling an exhaust-gas aftertreatment system.

[0002] A method and a device for controlling an exhaust-gasaftertreatment system are known from German Patent 199 03 439, where amethod and a device for controlling an exhaust-gas aftertreatment systemare described, the exhaust-gas aftertreatment system including at leastone catalytic converter, and a predefinable quantity of reducing agentbeing supplied to the exhaust-gas aftertreatment system as a function ofthe state of the internal combustion engine and/or the exhaust-gasaftertreatment system. The quantity of reducing agent supplied ispreferably determined on the basis of operating characteristics of theinternal combustion engine, such as the rotational speed and thequantity of fuel injected as well as variables which characterize thestate of the exhaust-gas aftertreatment system, e.g., the exhaust gastemperature upstream from, within, and/or downstream from theexhaust-gas aftertreatment system.

[0003] When using urea as the reducing agent in particular, metering ofthe reducing agent is problematical. If too little reducing agent issupplied to the exhaust-gas aftertreatment system, the result is thatnitrogen oxides are not adequately converted and then they enter theenvironment. If too much reducing agent is supplied, in particular inthe case of metering a urea solution, unwanted substances such as, e.g.,ammonia will enter the environment. This emission of ammonia must bereliably prevented. Therefore, the usual procedure tends to be to metertoo little reducing agent. In other words, the quantity of reducingagent is predefined so that tolerances in the area of the internalcombustion engine, the exhaust-gas aftertreatment system or the meteringsystem for metering the reducing agent do not result in the presence ofammonia in the exhaust gas.

ADVANTAGES OF THE INVENTION

[0004] By adjusting the quantity of reducing agent supplied, thequantity of reducing agent supplied may be set to yield only minimalnitrogen oxide emissions, with the emission of ammonia being reliablyprevented.

[0005] It is particularly advantageous if the adjustment is based on ameasured variable which characterizes the effect of the exhaust-gasaftertreatment system. Such a measured variable is supplied, forexample, by a sensor which detects nitrogen oxide emissions and/or asensor which detects ammonia emissions. In the case of systems whichwork with other reducing agents, other sensors will be used accordingly.

[0006] Sensors which detect measured values characterizing the effect ofthe exhaust-gas aftertreatment system are preferably used. Thesemeasured variables may depend on the concentration of varioussubstances. These are substances that are not converted at all or areconverted only partially in the exhaust-gas aftertreatment system,substances required for the reaction, and/or intermediates. It ispreferable to use sensors which detect substances which occur in theexhaust gas system downstream from the exhaust-gas aftertreatment systemand are necessary for the reaction in the exhaust-gas aftertreatmentsystem or are formed as intermediates during the reaction. It isparticularly advantageous to use a nitrogen oxide sensor whose differentvariants may be used for exhaust-gas aftertreatment. Furthermore, it maybe used for other functions in the area of control of the internalcombustion engine and/or the exhaust-gas aftertreatment system. In thecase of systems using urea as the reducing agent, an ammonia sensor isadvantageous.

[0007] It is particularly advantageous that adjustment is performed onlyin certain operating states of the internal combustion engine and/or theexhaust-gas aftertreatment system. In these particular operating states,the measured variable(s) is/are compared with values to be expected, andthe quantity of reducing agent is corrected on the basis of thiscomparison.

[0008] In a first embodiment, a first variable characterizing thequantity of ammonia in the exhaust gas downstream from the exhaust-gasaftertreatment system is detected, the quantity of reducing agent beingdecreased when the first variable exceeds an upper threshold valueand/or the quantity of reducing agent being increased when the firstvariable falls below a lower threshold value.

[0009] In a second embodiment, a second variable characterizing thequantity of nitrogen oxides in the exhaust gas downstream from theexhaust-gas aftertreatment system is detected, the quantity of reducingagent being decreased when the second variable falls below a lowerthreshold value and/or the quantity of reducing agent being increasedwhen the second variable exceeds an upper threshold value.

[0010] A particularly accurate control is obtained with an embodiment inwhich the quantity of reducing agent is decreased when the firstvariable exceeds an upper threshold value and/or the quantity ofreducing agent is increased when the second variable exceeds an upperthreshold value.

[0011] In addition, implementations in the form of a computer programhaving program code means and in the form of a computer program producthaving program code means are particularly important. The computerprogram according to the present invention has program code means forperforming all the steps of the method according to the presentinvention when the program is executed on a computer, in particular acontrol unit for an internal combustion engine of a motor vehicle. Inthis case, the invention is implemented by a program stored in thecontrol unit, so that this control unit, equipped with the program,constitutes the present invention in the same way as does the method forwhose execution the program is suitable. The computer program productaccording to the present invention has program code means which arestored on a computer-readable data medium for executing the methodaccording to the present invention when this program product is run on acomputer, in particular a control unit for an internal combustion engineof a motor vehicle. Thus, in this case, the invention is implemented bya data medium, so that the method according to the present invention maybe implemented when the program product and/or the data medium isintegrated into a control unit for an internal combustion engine of amotor vehicle in particular. The data medium and/or the computer programproduct may be in particular an electric memory medium, e.g., aread-only memory (ROM), an EPROM or a permanent electric memory such asa CD-ROM or DVD.

[0012] Advantageous and expedient embodiments and refinements of thepresent invention are characterized in the subclaims.

DRAWING

[0013] The present invention is explained in greater detail below on thebasis of embodiments illustrated in the drawing.

[0014]FIG. 1 shows a block diagram of the device according to thepresent invention;

[0015]FIG. 2 shows a detailed diagram of the device according to thepresent invention in the form of a block diagram; and

[0016]FIG. 3 shows a flow chart to illustrate the procedure according tothe present invention.

[0017]FIG. 1 shows the essential elements of an exhaust-gasaftertreatment system of an internal combustion engine. Internalcombustion engine 100 receives fresh air through a fresh air line 105.Exhaust gases from internal combustion engine 100 enter the environmentthrough an exhaust gas line 110. An exhaust-gas aftertreatment system115 is provided in the exhaust gas line. This is preferably a catalyticconverter. In addition, it is possible for a plurality of catalyticconverters to be provided for different pollutants, or for a combinationof at least one catalytic converter and one particle filter to be used.

[0018] In the preferred embodiment, exhaust-gas aftertreatment system115 includes one catalytic converter or three catalytic converters inwhich preferably three reactions take place. The aqueous urea solutionsupplied with control element 182 is converted to ammonia NH₃ in a firsthydrolysis catalytic converter 115 a. The actual reaction takes place indownstream SCR catalytic converter 115 b, where nitrogen oxides andammonia react to form nitrogen and water. Unconsumed ammonia is oxidizedin downstream oxidation catalytic converter 115 c.

[0019] In alternative embodiments in which other reducing agents areused, other catalytic converters may also be used or individualcatalytic converters may be omitted. The hydrolysis catalytic converterin particular may be omitted when the reducing agent is supplieddirectly.

[0020] In addition, a control unit 170 including at least one enginecontrol unit 175 and an exhaust-gas aftertreatment control unit 172 isalso provided. Engine control unit 175 sends triggering signals to afuel metering system 180. Exhaust-gas aftertreatment control unit 172exchanges signals with engine control unit 175. Furthermore, exhaust-gasaftertreatment control unit 172 sends triggering signals to an actuatorelement 182 situated in the exhaust gas line upstream from or in theexhaust-gas aftertreatment system.

[0021] In addition, various sensors which supply signals to theexhaust-gas aftertreatment control unit and the engine control unit mayalso be provided. For example, at least one first sensor 194 may beprovided to supply signals characterizing the state of the internalcombustion engine. A second sensor 177 supplies signals characterizingthe state of fuel metering system 180.

[0022] A temperature sensor 191 detects a temperature variable T whichcharacterizes the temperature of the exhaust-gas aftertreatment system.Temperature sensor 191 is preferably situated downstream from catalyticconverter 115. A sensor 193 preferably detects a nitrogen oxideconcentration in the exhaust gas downstream from the exhaust-gasaftertreatment system. An emission sensor 192 detects an NH₃ signalwhich characterizes the quantity of ammonia in the exhaust gasdownstream from the exhaust-gas aftertreatment system. It isparticularly advantageous if the emission sensor is situated between SCRcatalytic converter 115 b and oxidation catalytic converter 115 c. Anemission sensor 192 or a sensor 193 which detects the nitrogen oxideconcentration in the exhaust gas downstream from the exhaust-gasaftertreatment system is preferably provided.

[0023] Exhaust-gas aftertreatment control unit 172 preferably receivesthe output signals of sensors 191, 192 and 193. Engine control unit 175preferably receives the output signals of second sensor 177. Othersensors not shown here may also be provided to characterize a signalrelated to the driver's intent or other ambient conditions or engineoperating states.

[0024] It is particularly advantageous if the engine control unit andthe exhaust-gas aftertreatment control unit form one structural unit.However, it is also possible for these units to be designed as twophysically separate control units.

[0025] The procedure according to the present invention is describedbelow using the example of a reduction catalytic converter, which isused in particular with direct-injection internal combustion engines.However, the procedure according to the present invention is not limitedto this application but instead may also be used with other internalcombustion engines having an exhaust-gas aftertreatment system. Inparticular, it may be used with exhaust-gas aftertreatment systems inwhich a catalytic converter and a particle filter are combined.

[0026] On the basis of the sensor signals received, engine control 175computes trigger signals to be applied to fuel metering system 180,which then meters the proper quantity of fuel for internal combustionengine 100. During combustion, nitrogen oxides may be formed in theexhaust gas. They are converted into nitrogen and water by reductioncatalytic converter 115 b in exhaust-gas aftertreatment system 115. Todo so, a reducing agent must be supplied to the exhaust gas upstreamfrom the exhaust-gas aftertreatment system. In the embodimentillustrated here, this reducing agent is supplied to the exhaust gas viaactuator member 182. Ammonia is preferably used as the reducing agent;it is formed from a urea solution in hydrolysis catalytic converter 115a.

[0027] Actuator element 182 is preferably situated in exhaust gas line110. However, it may also be mounted in or on the exhaust-gasaftertreatment system, in particular on hydrolysis catalytic converter115 a.

[0028] In the embodiment described below, an aqueous urea solution issupplied to the exhaust-gas aftertreatment system with actuator element182. The aqueous urea solution is hereinafter referred to as thereducing agent.

[0029]FIG. 2 shows exhaust-gas aftertreatment control unit 172 ingreater detail. Elements already described in conjunction with FIG. 2are labeled here with the same reference numbers. Essentially,exhaust-gas aftertreatment control unit 172 includes a signalpreselection unit 200 and a correction value determination unit 220.Output signal H0 of signal value preselection goes to a node 210 alongwith output signal K of the correction value preselection. Actuatorelement 182 is triggered with output signal H of node 210.

[0030] Correction value determination unit 220 processes the outputsignals of sensor 193, which detects the nitrogen oxide concentration inthe exhaust gas downstream from the exhaust-gas aftertreatment systemand/or the output signal of emission sensor 192, which supplies an NH₃signal that characterizes the quantity of ammonia in the exhaust gasdownstream from the exhaust-gas aftertreatment system. Furthermore,correction value determination unit 220 receives operatingcharacteristics such as rotational speed N and quantity of fuel QKinjected into the internal combustion engine.

[0031] Signal preselection unit 200 receives various operatingcharacteristics, e.g., rotational speed N, quantity of fuel QK injectedinto the internal combustion engine, and various temperature variablesT, which characterize in particular the exhaust gas temperature upstreamfrom, within and/or downstream from exhaust-gas aftertreatment system115.

[0032] Based on these variables, the signal preselection unit calculatesa triggering signal H0 with which actuator element 182 is triggered.Values H0 which determine the triggering signal for actuator element 182are preferably stored in one or more engine characteristics maps as afunction of the input variables. It is possible to provide for variablesstored in the engine characteristics map to be corrected on the basis ofvarious operating characteristics.

[0033] A procedure for preselecting triggering signal H0 is described inGerman Patent 199 03 439, for example. However, the procedure accordingto the present invention is not limited to this type of determination ofthe triggering signals for actuator element 182. It may also be appliedin a similar manner to other procedures for determining the triggeringsignal or for determining other variables which determine thistriggering signal. It is essential that signal preselection unit 200preselects the triggering signal for actuator element 182 and/or avariable which determines the quantity of reducing agent to be suppliedto the exhaust-gas aftertreatment system.

[0034] Since internal combustion engines and/or exhaust-gasaftertreatment systems usually exhibit tolerances, the values stored insignal preselection unit 200 are very inaccurate, i.e., are also subjectto high tolerances. In other words, compromises must be made with regardto emissions of ammonia and/or nitrogen oxides.

[0035] To be able to achieve a further reduction in emissions and/orunwanted exhaust gas constituents, a specific adjustment of the meteringsystem to the particular internal combustion engine and/or to theparticular exhaust-gas aftertreatment system is performed according tothe present invention. To do so, sensor 192 situated in or downstreamfrom the exhaust-gas aftertreatment system is used to detect nitrogenoxide emissions or ammonia emissions.

[0036] The emissions thus detected are compared with suitable setpointvalues which are to be achieved. The values stored in the signalpreselection unit are corrected when limiting values are exceeded and/ornot met. This is accomplished by correction value preselection unit 220,which preselects a correction value K for the embodiment illustratedhere, this correction value being associated with the output signal H0of signal preselection unit for forming triggering signal H. An additiveor multiplicative correction is preferably performed.

[0037] This compensation is preferably performed at the time of initialoperation of the vehicle and then at regular intervals and/or whencertain states of the internal combustion engine and/or exhaust-gasaftertreatment system occur.

[0038]FIG. 3 illustrates the functioning of correction valuedetermination unit 220 in greater detail. In a first step 310, theoperating point of the internal combustion engine and/or the exhaust-gasaftertreatment system is detected. For example, rotational speed N andquantity of fuel injected QK of the internal combustion engine areanalyzed. Furthermore, it is possible to provide for the reading of anelapsed time meter and/or an odometer which detects the total distancetraveled.

[0039] Then in step 320 a check is performed to ascertain whether theprevailing state is one in which adjustment is possible and appropriate.Such operating states include in particular steady-state operatingstates in which the values detected are constant over a certain periodof time. This is necessary because the exhaust-gas aftertreatment systemhas a relatively long dead time. This means that when there are changesin the operating state, they have an effect only after a certain lagtime.

[0040] An adjustment is performed only in such operating states in whichthe system is in a stable state, i.e., a certain period of time shouldhave elapsed since the last change in operating characteristics. Inaddition, it is possible to provide for the adjustment to take placeonly when the internal combustion engine and/or the exhaust-gasaftertreatment system is at certain operating points. These operatingpoints are defined by various operating characteristics, e.g.,rotational speed N of the internal combustion engine, quantity of fuelQK injected and/or certain temperature values T, in particular for theexhaust gas temperature.

[0041] In addition, it is possible to check on whether a certain periodof time has elapsed since the last adjustment and/or whether a certaindriving performance of the internal combustion engine and/or theexhaust-gas aftertreatment system has been achieved.

[0042] If query 320 detects that a suitable operating state does notexist, the program is terminated in step 325. If a suitable operatingstate prevails, then in step 330 the output signal of suitable sensorsis analyzed. Measured variables characterizing the effect of theexhaust-gas aftertreatment system are preferably detected here. Forexample, in the case of the supply of urea, the ammonia content in theexhaust gas downstream from the exhaust-gas aftertreatment system, inparticular downstream from the reduction catalytic converter, may beanalyzed. In addition, it is possible for the nitrogen oxideconcentration, i.e., the concentration of unwanted substances that areto be eliminated by the exhaust-gas aftertreatment system, to bedetected by a sensor.

[0043] In the next step 340, a lower threshold value US is preselectedas a function of the operating state of the internal combustion engineand/or the exhaust-gas aftertreatment system. Similarly, an upperthreshold value OS is also preselected as a function of these variables.The state variable is preferably the rotational speed, quantity of fuelinjected QK and, if necessary, other variables such a temperature valuesT characterizing the exhaust gas temperature.

[0044] In a simplified embodiment, it is possible to provide for fixedvalues to be specified, this method being implemented only at certainoperating points at which the NH₃ and NO_(x) signals assume the expectedvalues.

[0045] Then in step 350 a check is performed to determine whether theoutput signal of the ammonia sensor is greater than the upper thresholdvalue. If this is the case, then in step 360 a correction value isdefined, resulting in a decrease of the quantity of reducing agentsupplied. For example, it is possible to provide for a negativecorrection value to be supplied or for the output value of the signalpreselected to be multiplied by a value less than 1. Then the programlikewise ends in step 325.

[0046] If query 350 detects that the output signal of ammonia sensor 192is lower than the upper threshold value OS, then subsequent query 370checks on whether the output signal of the sensor is smaller than lowerthreshold value US. If this is the case, then in step 380 a correctionvalue is preselected so that it produces an increase in the quantity ofreducing agent supplied.

[0047] In the alternative embodiment, it is possible to provide for acheck to be performed in step 370 to determine whether the NO_(x) outputsignal of a nitrogen oxide sensor 193 is greater than a threshold valueUS, in which case a correction is also performed to increase thequantity of reducing agent. Accordingly, it is also possible to proceedin the case of query 350 such that the output signal of a NO_(x) sensoris checked to determine whether it is lower than a threshold value andin this case the quantity of reducing agent is decreased.

[0048] According to the present invention, in certain states of theinternal combustion engine and/or the exhaust-gas aftertreatment system,the measured variable characterizing the effect of the exhaust-gasaftertreatment system is compared with expected values and the quantityof reducing agent is corrected on the basis of this comparison.

[0049] It is possible for the output signal of the signal preselectionunit to be corrected or for the signal preselection to be altered, i.e.,the values stored in the engine characteristics maps, for example, to bealtered accordingly. In particular, the quantity of reducing agent isdecreased when the quantity of ammonia exceeds an upper threshold valueand/or the quantity of reducing agent is increased when the quantity ofnitrogen oxide exceeds an upper threshold value.

[0050] According to the present invention, systems having only oneammonia sensor or one nitrogen oxide sensor are used, but as analternative it is also possible to provide systems with an ammoniasensor and a nitrogen oxide sensor. It is particularly advantageous touse a nitrogen oxide sensor which may also be used for other functionsin the area of controlling the internal combustion engine and/or theexhaust-gas aftertreatment system.

[0051] The procedure according to the present invention is not limitedto systems which use urea or a similar compound as a reducing agent, inparticular when a nitrogen oxide sensor is used, but instead it may alsobe used with other systems which employ different reducing agents. Inparticular, it is possible to provide for hydrocarbons to be supplied asreducing agents to the exhaust gas. This is possible in particular usingan actuator member 182. As an alternative, it is also possible to meterhydrocarbons, which are used in particular as fuel supplied viaconventional actuator elements 180 for controlling the quantity of fuelinjected into the internal combustion engine. Thus, for example, it ispossible to provide for a secondary injection to introduce thecorresponding hydrocarbons into the exhaust gas.

What is claimed is:
 1. A method for controlling an exhaust-gasaftertreatment system, in particular for an internal combustion engine,the exhaust-gas aftertreatment system comprising at least one catalyticconverter, a preselectable quantity of reducing agent being supplied tothe exhaust-gas aftertreatment system as a function of the state of theinternal combustion engine and/or the exhaust-gas aftertreatment system,wherein the quantity of reducing agent supplied is adjusted.
 2. Themethod as recited in claim 1, wherein a measured variable characterizingthe effect of the exhaust-gas aftertreatment system is detected.
 3. Themethod as recited in one of the preceding claims, wherein the measuredvariable detected depends on the concentration of substances which areonly partially converted or are not converted at all in the exhaust-gasaftertreatment system, the concentration of substances required for theconversion, and/or the concentration of unwanted intermediates formed.4. The method as recited in one of the preceding claims, wherein themeasured variable is compared with expected values in certain states ofthe internal combustion engine and/or the exhaust-gas aftertreatmentsystem, and the quantity of reducing agent is corrected on the basis ofthis comparison.
 5. The method as recited in one of the precedingclaims, wherein a first variable, characterizing the quantity of ammoniain the exhaust gas downstream from the exhaust-gas aftertreatmentsystem, is detected, the quantity of reducing agent being decreased whenthe first variable exceeds an upper threshold value, and/or the quantityof reducing agent being increased when the first variable falls below alower threshold value.
 6. The method as recited in one of the precedingclaims, wherein a second variable, which characterizes the quantity ofnitrogen oxides in the exhaust gas downstream from the exhaust-gasaftertreatment system, is detected, the quantity of reducing agent beingdecreased when the second variable falls below a lower threshold valueand/or the quantity of reducing agent being increased when the secondvariable exceeds an upper threshold value.
 7. The method as recited inone of the preceding claims, wherein the quantity of reducing agent isdecreased when the first variable exceeds an upper threshold value,and/or the quantity of reducing agent is increased when the secondvariable exceeds an upper threshold value.
 8. A device for controllingan exhaust-gas aftertreatment system, in particular for an internalcombustion engine, the exhaust-gas aftertreatment system comprising atleast one catalytic converter, a predefinable quantity of reducing agentbeing supplied to the exhaust-gas aftertreatment system as a function ofthe state of the internal combustion engine and/or the exhaust-gasaftertreatment system, wherein means are provided for adjusting thequantity of reducing agent supplied.
 9. A computer program havingprogram code means for performing all steps of any of claims 1 through11 when the program is executed on a computer, in particular a controlunit for an internal combustion engine.
 10. A computer program producthaving program code means stored on a computer-readable data medium forperforming the method as recited in any of claims 1 through 11 when theproduct is executed on a computer, in particular a control unit for aninternal combustion engine.